1. Field of the Invention
This invention relates to photographic products and processes and particularly to diffusion transfer photographic products and processes.
2. Description of the Prior Art
Diffusion transfer photographic products and processes are known to the art and details relating to them can be found in U.S. Pat. Nos. 2,983,606; 3,415,644; 3,415,645; 3,415,646; 3,473,925; 3,482,972; 3,551,406; 3,573,042; 3,573,043; 3,573,044; 3,576,625; 3,576,626; 3,578,540; 3,569,333; 3,579,333; 3,594,164; 3,594,165; 3,597,200; 3,647,437; 3,672,486; 3,672,890; 3,705,184; 3,752,836; 3,857,865; and British Pat. No. 1,330,524.
Essentially, diffusion transfer photographic products and processes involve film units having a photosensitive system including at least one silver halide layer usually integrated with an image-providing material. After photoexposure, the photosensitive system is developed to establish an imagewise distribution of a diffusible image-providing material, at least a portion of which is transferred by diffusion to an image-receiving layer capable of mordanting or otherwise fixing the transferred image-providing material. In some diffusion transfer products, the transfer image is viewed by reflection after separation of the image-receiving element from the photosensitive system. In other products however, such separation is not required and instead, the transfer image in the image-receiving layer is viewed against a reflecting background usually provided by a dispersion of a white, light-reflecting pigment--such as titanium dioxide.
Diffusion transfer photographic processes and products providing a dye image viewable against a reflecting background without separation are oftentimes referred to in the art as, "integral negative-positive film units" and such units are of two general types. Integral negative-positive film units of the first type as described, for example, in the above-noted U.S. Pat. No. 3,415,644 include appropriate photosensitive layer(s) and image dye-providing materials carried on an opaque support, an image-receiving layer carried on a transparent support and means for distributing a processing composition between them. Photoexposure is made through the transparent base and image-receiving layer and a processing composition which includes a reflecting pigment is distributed between the image-receiving and photosensitive components. After distribution of the processing composition and before processing is complete, the film unit can be--and usually is--transported into light. Accordingly, in integral negative-positive film units of this type the reflecting pigment-containing layer must be capable of performing specific and critical assigned functions. Until processing is complete, for example, the distributed reflecting layer must be able to provide at least partial protection against further exposure of the photoexposed element but at the same time permit transfer of the image dyes through the layer of processing composition and light-reflecting pigment to the image-receiving layer. After transfer, the layer must provide a suitably efficient reflecting background for viewing the dye image transferred to the image-receiving layer since the image is viewed through the same side of the film unit as exposure was effected.
Integral negative-positive film units of a second type, as described for example, in U.S. Pat. No. 3,594,165, include a transparent support, carrying the appropriate photosensitive layers and associated image dye-providing materials, a permeable opaque layer, a permeable layer containing a light-reflecting pigment, an image-receiving layer viewable through the transparent support against the light-reflecting layer, and means for distributing a processing composition between the photosensitive layer and a transparent cover or spreader sheet. Additionally, integral negative-positive film units of this second type have means for providing an opaque processing composition to provide a second opaque layer after photoexposure to prevent additional exposure of the photosensitive element. In film units of this second type, exposure is made through the transparent cover sheet. After distribution of the processing composition and installation of the second opaque layer, this type of film unit can also be transported into light before processing is complete. Accordingly, in film units of this second type, the reflecting pigment-containing layer may also perform the critical assigned functions of providing at least partial protection for the photoexposed element until processing is complete without interfering with transfer of the image dyes. Also after transfer is complete, the layer must provide a suitable reflecting background for viewing the dye image transferred through the reflecting pigment-containing layer.
In view of the above, the performance characteristics desired of reflecting pigments employed in integral negative-positive film units can be said to be reasonably well defined. When employed in film units of the first type, they must be compatible with and dispersible in the processing composition so that upon distribution, a layer can be provided presenting the requisite degree of opacity for the photoexposed element as well as a suitable degree of permeability for effective dye transfer. Likewise, when employed in film units of the second type, the pigments ideally should be capable of being effectively dispersed in selected polymeric matrix materials to provide coating dispersions adaptable to high speed, high volume coating techniques involved in commercial film manufacturing processes, and to provide uniform layers having the capability of providing the requisite opacity and permeability dicussed above. Additionally, when used in either type of film unit, the optical properties of the pigment as well as the layer containing it must be such so as to present an efficient light-reflecting background for viewing the dye image against it.
As mentioned, reflecting layers of integral negative-positive film units known to the art have usually employed titanium dioxide as the reflecting pigment most nearly meeting the requisite performance characteristics discussed above. Particularly useful titanium dioxides have been commercially available, generally spherical, rutile titanium dioxides having an average particle size of about 0.2 microns.